Magnetic sensors are widely used in a variety of industrial fields, e.g. position sensing and also including for example, medicine. Magnetic sensors typically convert a magnetic flux into an electrical signal, either in the forms of a differential voltage or a differential current. Typical magnetic field sensors available, include hall effect devices and magnetic field effect transistors (MagFETs). A significant advantage of MagFETs over Hall effect devices is that they can be formed as a part of a standard integrated circuit, for example using CMOS processes. Hall effect devices are typically manufactured as discrete devices.
FIG. 1 is an illustration of a typical dual drain MagFET 10, consisting of a source 11, gate 14, and two drains D112 and D213. In operation, a suitable bias voltage 15, is applied to the gate 14, and a second biasing arrangement causes a current (I) to flow between the source 11, and the two drains 12, 13. Assuming ideal devices and in the absence of a magnetic flux, the currents flowing through the first drain (D1) 12, and second drain (D2) 13, would be equal. When a magnetic field is present a Lorentz force is exerted upon the moving charge carriers in the MagFET. The direction of exertion of the force is determined by reference to the well known right hand rule. In the device shown in FIG. 1, a magnetic field flowing transverse to the page from top to bottom would result in a force acting upon the charge carriers in the MagFET such that the current is diverted in a clockwise direction. Whereas a magnetic field flowing in the opposite direction would result in a force acting in an anticlockwise direction. Such a force will have the effect of deflecting the path of charge carriers from one drain to the other. This deflection causes a differential between the currents of the first drain and second drain. This differential current may be amplified to produce an indication or measurement of the magnetic field.
When operating MagFETs at low currents (100 nA level) offset and noise become very significant. For example, if a device having an aspect ratio of one is operated at 100 nA, it has an enhancement voltage of only about 100 mV. The differential introduced in such a device by (for example) a 10 Gauss magnetic field may only be about 20 ppm. The voltage excursion on the gate required to produce a similar change in current is only of the order of a microvolt or less. Given that noise and offsets at the gate may be of the order of tens of millivolts, the signal arising from the magnetic field is not easy to detect. The effects of offset may be alleviated using calibration, as the offset tends to stay constant for a reasonable amount of time. However, the offset changes with temperature which necessitates the use of special temperature compensating circuitry to ensure that any change in temperature does not require re-calibration.
The technique of chopping is frequently used to reduce errors arising from noise and offset errors in measuring circuits. Unfortunately, standard chopping techniques cannot be used with MagFETs as the technique of chopping depends on the devices that make up the inputs being completely interchangeable, which is not the case for MagFETs. In a MagFET the magnetic flux will create different offsets depending on the relative orientation of the MagFET, i.e. the flux effectively identifies which is the right hand and which is the left hand gate. In other words, the magnetic field and its effects are vector quantities and it is the orientation of the MagFET to the magnetic field which defines the inputs. In order to chop the inputs of a MagFET, the physical orientation of the MagFET with respect to the magnetic field would have to change, at a rate in excess of the rate of change of the Magnetic Field. It will be appreciated by those skilled in the art that this is not a practicable solution.
U.S. Pat. No. 5,801,533 describes a typical circuit arrangement for implementing a dual drain MagFET magnetic field measurement device circuit. As explained above, this circuit cannot use conventional prior art chopping techniques.
An alternative MagFET device to the dual drain MagFET is the triple drain MagFET, an example of which is shown in FIG. 2. The triple drain MagFET comprises a gate 22, a source 20, a central drain 24 and two side drains 23,25 disposed at either side of the central drain 24. The central drain 24 is larger than the two side drains 23,25. This device operates in a similar fashion to the dual drain version, except the differential current is measured between the two side drains 23,25. In use, the majority of the current 28 flows through the central drain D2, with only a small portion 27,29 flowing in the two side drains (D1 and D3). In the presence of a magnetic field flowing transverse to the surface of the MagFET, the current flowing 27,29 in one side drain 23,25 will increase and the current flowing 29,27 in the other side drain 25,23 will decrease. The current 28 in the main drain 24 will remain substantially the same. A measurement of the differential current (27-29) between the two side drains may used to produce an indication or measurement of the magnetic field.
It will be appreciated that the relative change in measured current from a triple drain MagFET is typically greater than for dual drain MagFETs. Although the triple drain MagFET typically has superior signal to noise characteristics to dual drain MagFETs, it still suffers to a large extent from the problems described above of noise and offset.
Baltes H. P. and Popovic R. S., “Integrated Semiconductor Magnetic Field Sensors”, Proceedings of the IEEE, Vol. 74, No. 8 August 1986, discloses a Hall effect plate having a plurality of contacts, wherein, performance is improved by the combination of Hall and magneto-resistance effects. The problem with this device is that chopping techniques cannot be used and the Hall effect plate cannot be constructed on an integrated circuit as easily as a MagFET.
As described above, in a MagFET, a magnetic field will induce a differential signal corresponding to an indication of the magnetic strength of a field it is placed in that will look exactly the same as one introduced by a device mismatch induced offset. Accordingly, prior art chopping techniques, which reduce standard offset and noise, will similarly reduce the magnetically induced offset.
It would be advantageous therefore if it were possible to distinguish magnetically induced offsets from mismatch and noise induced offsets and to produce a circuit that can detect magnetic fields below the nominal noise floor of the sensing device. In particular, it would be advantageous if a circuit could be provided which reduces the effects of low frequency noise in magnetic field measurement circuits using a MagFET as the sensor.